The present invention relates to a brushless motor that is mainly employed as an air blower in an air conditioning system for vehicles.
Brushless motors in the prior art include the one disclosed in Japanese Unexamined Utility Model Publication No. H2-139473. This brushless motor is provided with a rotatable rotor having field magnets, a stator that generates a rotating magnetic field for the rotor and a means for excitation that excites the stator. Thus, in this brushless motor, a rotating magnetic field is generated by sequentially exciting a plurality of arc-shaped end portions at the stator in the direction of the radius with the means for excitation, and the rotor rotates as the magnets provided at the rotor are repeatedly attracted and repulsed relative to the rotating magnetic field.
In this brushless motor, the means for excitation is constituted of an exciting coil wound around the stator core and a means for switching that controls the direction of the current running to the exciting coil. Normally, a plurality of field effect transistors (FETs) are employed to constitute the means for switching. These FETs control the direction of the current running to the exciting coil by controlling the signal applied to the gate terminal of each FET. Since a relatively large current is handled by the FETs, generating a great deal of heat, often reaching up to approximately 150.degree. C., the FETs are provided with a heat sink. Under normal circumstances, such a heat sink will be constituted of aluminum or an aluminum alloy to ensure that the desired thermal conductivity, weight and so forth are achieved and because of the ease with which heat sinks can be formed using these materials.
However, when the brushless motor described above is mounted at an air conditioner control apparatus for vehicles that is operated in a cold area or the like, dry snow and dust particles that are drawn in along with the outside air through the external air induction port of the vehicle may become electrically charged due to friction generated at the fan of the brushless motor. These charged particles then come in contact with the heat sink exposed through the case housing to electrostatically charge the heat sink. If the voltage resulting from this static electricity reaches approximately 7 KV, it may be discharged to the field effect transistor on the control board, destroying the FET since the typical withstand voltage of a FET is approximately 5 KV.